There are many problems associated with combining video signals with computer graphics signals, but probably the most significant problem is that of synchronising the video and graphics signals so that correct placement of the signals occurs on the combined display.
A method of synchronising the computer graphics signal and the video signal is, for example, known from U.S. Pat. No. 4,599,611. In this method, the computer graphics system takes all its timing signals from the sync signals of the video source. Thus the graphics and video signals are accurately aligned relative to one another. The combined video output signal is also therefore based on the video source timing.
However, such a system limits the graphics display modes in which the computer can operate to only those modes that are compatible with the video source, ie. that have the same number of horizontal display lines per full screen of display, and that have a line rate substantially the same as that of the video source. This generally restricts the available graphics modes to low resolution modes, thereby not making full use of the graphics capability of the computer.
A further problem is that the aspect ratio for the computer graphics is usually not the same as that for usual standards of video image. Correction of the graphics aspect ratio relative to that of the video cannot easily be achieved when the computer graphics are synchronised to the video source, and hence in the combined display the computer graphics will be distorted. A circle drawn in computer graphics will appear as an ellipse in the combined display. This may limit the usefulness of the computer graphics to that of adding alphanumeric text to the video picture.
A further constraint imposed by synchronising by a generator sync for the computer graphics to the video source is that both the graphics and the video displays necessarily fill the whole of the display area on the screen.